Preserving knowledge has always been a challenge because people die. Books, stories, and songs were early attempts at a solution. The Internet Archive is another attempt. It seems likely it is impossible to preserve all useful knowledge forever so we're just going to have to keep reinventing the wheel. As you get older you too will see knowledge disappear as the old leave the world. It's already apparent that a lot of kids are repeating common cliche phrases without actually knowing what they mean. Some current knowledge is not widely known and could easily be destroyed (how to make state of the art integrated circuits for example). Documentation is often insufficient. In 50-100 years no one may know how to make a cellphone (and perhaps won't care.) Some technologies may become magic, we'll use them but we won't know how they work and when the devices are gone the magic will be gone. There is already some concern that when the USA gets around to rebuilding infrastructure we won't know how to rebuild some things. There were a lot of custom construction techniques in the old days. Advanced proprietary chemistry seems ripe for this kind of degradation, in the same way that custom sensors are. There are other things that NASA has lost the capability of doing as engineers retire. To some extent this is a hard limit on the ability of civilization on Earth to advance. There is only so much knowledge that can be kept current at any one time. A reduction in world population would likely mean a reduction in the perpetuation of knowledge. On the other hand, if everyone on Earth was educated and trained that would likely cause a knowledge explosion.